As Tim states, there are different opinions on the issue. Since I just finished the rebuild on my R10, I asked this question to my friend (whom I trust completely) about it.
He said, as Tim states, that GM recommends SAE 90. He said that's fine for non-synchronized transmissions. BUT for synchronized transmissions, people may be recommending something else.
Since mine is non-synchro, I'll be using SAE 80-90.
~ Victor
1941 3/4-Ton Pickup (in process). Read about it
in the DITY Gallery1955 Grumman Kurbside "Doughboy" 235/3 on tree w/ OD
1957 3100 - moved on
1959 C4500 Short Bus "Magic Bus" - moved on
1959 G3800 1 Ton Dually "Chief" - moved on
1958 C4400 Viking "Thor" ~
moved on to fellow Bolter 
